ALPS: An Adaptive Learning, Priority OS Scheduler for Serverless Functions

FaaS (Function-as-a-Service) workloads feature unique patterns. Serverless functions are ephemeral, highly concurrent, and bursty, with an execution duration ranging from a few milliseconds to a few seconds. The workload behaviors pose new challenges to kernel scheduling. Linux CFS (Completely Fair Scheduler) is workload-oblivious and optimizes long-term fairness via proportional sharing. CFS neglects the short-term demands of CPU time from short-lived serverless functions, severely impacting the performance of short functions. Preemptive shortest job first-shortest remaining process time (SRPT)-prioritizes shorter functions in order to satisfy their short-term demands of CPU time, and therefore, serves as a best-case baseline for optimizing the turnaround time of short functions. A significant downside of approximating SRPT, however, is that longer functions might be starved.

In this paper, we propose a novel application-aware kernel scheduler, ALPS (Adaptive Learning, Priority Scheduler), based on two key insights. First, approximating SRPT can largely benefit short functions but may inevitably penalize long functions. Second, CFS provides necessary infrastructure support to implement user-defined priority scheduling. To this end, we design ALPS to have a novel, decoupled scheduler frontend and backend architecture, which unifies approximate SRPT and proportional-share scheduling. ALPS' frontend sits in the user space and approximates SRPT-inspired priority scheduling by adaptively learning from an SRPT simulation on recent past workload. ALPS' backend uses eBPF functions hooked to CFS to carry out the continuously learned policies sent from the frontend to inform scheduling decisions in the kernel. This design adds workload intelligence to workloadoblivious OS scheduling while retaining desirable properties of OS schedulers. We evaluate ALPS extensively using two production FaaS workloads (Huawei and Azure) and results show that ALPS achieves a reduction of 57.2% in average function execution duration, compared to CFS.